Watercraft are often powered by outboard motors positioned at the stern of the craft. These motors have an internal combustion engine positioned within a cowling of the motor. For among other reasons, because the outboard motor is positioned at the stern of the craft, and because the motor is tiltable, it is desirable to keep the engine's size and weight to a minimum.
In order to keep the engine small, its various parts are typically mounted much closer to one another than might be the case with similar types of engines utilized in other settings. One problem which arises in these engines relates to keeping various of the components of the engine cool when they are so close to one another. The cooling problem is further aggravated by the fact that the engine is positioned within an enclosed cowling, trapping the heat therein.
One engine feature which is especially difficult to keep cool is the exhaust manifold. If this feature of the engine is not adequately cooled, large quantities of heat are transferred from the exhaust gases passing therethrough to other portions of the engine. This heat may damage other engine components. In particular, if heat is transferred from the exhaust gases to the combustion chambers, the combustion efficiency of the engine is greatly reduced, lessening engine power output. In addition, if the combustion chambers become too hot, the lubricating oil may be scorched and burned, reducing its effectiveness. The cylinder walls, pistons or rings may also be warped or damaged.
Some mechanisms have been proposed for cooling these exhaust manifolds, including positioning them in a cooling water jacket. The use of a cooling water jacket greatly complicates the exhaust and cooling system, however, and adds weight and size to the engine.
A cooling arrangement for an exhaust manifold which overcomes the problems associated with the prior art, is desirable.